Structure and dynamics of the radical cation of ethane arising from the Jahn–Teller and pseudo-Jahn–Teller effects

Abstract

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of C₂H₆ has been recorded in the region of the adiabatic ionization threshold. The partially rotationally resolved spectrum indicates the existence of several vibronic states of C₂H₆⁺ with less than 600 cm⁻¹ of internal excitation. The analysis of the rotational structures assisted by ab initio calculations enabled the determination of the adiabatic ionization energy of C₂H₆ and the investigation of the structure and dynamics of C₂H₆⁺ at low energies. The ground state of C₂H₆⁺ is found to be a ²A_g state of diborane-like structure with strongly mixed (a_1g)⁻¹ and (e_g)⁻¹ configurations. The vibrational structure reveals the importance of large-amplitude nuclear motions involving the diborane distortion modes, the C–C stretching motion, and the internal rotation at elongated C–C distances. The spectrum is analyzed in the light of the information obtained in earlier studies of C₂H₆⁺ by ab initio quantum chemistry, EPR spectroscopy and photoelectron spectroscopy.